This invention generally relates to color picture tubes and, more particularly, to a method and apparatus for affixing tension masks to mask frames for a color picture tube.
A conventional color picture tube comprises a glass faceplate panel, a funnel attached to the panel by a glass frit, and an electron gun within the neck of the funnel for emitting red, green, and blue electron beams. An aperture mask, or otherwise known as a shadow mask, which may be either a formed mask or a tension mask, is interposed between the gun and the screen and secured to the panel by a shadow mask frame. The electron beams emitted from the electron gun pass through the apertures in the shadow mask and strike a phosphor screen applied to the inner surface of the panel. The electron beams cause the phosphors to emit light so that an image is displayed on the panel.
A shadow mask is a thin sheet of metal, such as steel, that is contoured to somewhat parallel the inner surface of the tube faceplate. A tension mask can be a strand tension mask, tiebar tension mask or a tension focus mask. A tension focus mask comprises an array of strands and perpendicular crosswires overlying the strands and separated by an insulator. The strands and crosswires form the apertures in the mask through which electron beams emitted from the electron gun pass. Different voltages are applied to the strands and crosswires to create multiple focusing lenses in each of the mask apertures. Generally, in a tension focus mask, the strands are vertically oriented and supported in tension by the shadow mask frame.
In tension mask assemblies, because the mask must be affixed to the frame under tension and the mask strands must be positioned such that they are parallel and uniformly spaced from each other, it is desirable to employ a jig or fixture during the manufacture of the mask frame assembly to attain the correct tensioning and strand spacing. Ideally, the jig or fixture would secure a mask frame; thus, allowing a tension mask to be positioned and aligned, after which the tension mask would be formed to the mask frame and welded in place. Such a jig or fixture is desirable in manufacturing because it allows mask frame assemblies to be assembled in a repetitively precise manner.
The present invention provides a method and apparatus for precisely welding a tension mask to a mask frame. The apparatus includes a base clamping plate element upon which is disposed a set of clamps for holding a mask frame, a forming element is included to form a tension mask to a mask frame, a frame deflector is also included in the apparatus to deflect the vertical component of the mask frame to a desired position under clamping pressure, and finally, a welding port aperture is disposed within the mask forming element. The method of the invention includes clamping a mask frame to the clamping plate element and then engaging a frame deflector to deflect the mask frame to a desired position under clamping pressure. The tension mask is then aligned to the mask frame. The mask forming element is then engaged and forms the tension mask around the vertical component of the mask frame. After the tension mask has been formed to the mask frame, the tension mask is securely held in place and welded to the mask frame by tack, seam or spot welding so as to securely affix the mask to the mask frame. After the assembly has sufficiently cooled from welding, all clamps are removed and the completed mask frame assembly is removed from the fixture.